The field of radio frequency photonics, which is the combination of microwave engineering with photonic technology plays a major role in several applications like wireless communication, ultra-fast signal processing, spectroscopy, metrology, etc. Radio-over-Fiber (RoF) technique is playing vital role in attaining the goals of various applications. The RoF is a technique where analog radio frequency signals are transmitted over fiber, in which optical signal is modulated by radio frequency of several GHz (Torres‐Company & Weiner, 2014). As the world population increases, crave for internet also increases. In order to meet this kind of situation, a low cost and highly effective technique must be needed. Optical Flat Comb Spectrum (OFCS) with multiple frequencies can be the possible solution for emerging bandwidth hungry applications. OFCS technique applies on high data rate applications, where each channel will have same bit rate (Nesset, 2015). To enhance energy efficiency in the total transmission model and to reduce number of central office sites, OFCS will be the better way. Different kinds of techniques to generate ultra-dense flat comb spectrum including the use of phase modulators and mode locked lasers have been reported (Jung et al., 2011; Zhang, et al., 2011; Vujicic, Anandarajah, Browning, & Barry, 2013). But the power difference between the spectral lines is larger. Even though the mode locked lasers produce super-continuum spectrum, they have instability and drawbacks in tunability . In non-linear medium, OFCS can also be generated using Four Wave Mixing (FWM) (Cruz, 2008). But still the flatness in the comb cannot be obtained due to low power efficiency of FWM. If FWM for OFCS, the main drawback is that it cannot be used effectively for long distance fiber (Cruz, 2008). Whereas the phase modulator is incapable of power handling, so spectral lines generated by PM will have inequalities. The combination of Polarization modulator (Polm) and Brillouin Assisted Power Equalizer (BAPE) also produces flattened optical comb spectrum (Li et al., 2014). However, the spacing between the spectral lines is fixed and separate frequency tunable optoelectronic oscillator is referred. A nonlinear mode resonator pumped with continuous wave laser has been used to produce OFCS, but it results in instability of fiber and to avoid this microphotonic structure has been included externally to maintain high Q-factor . In paper Hraghi, Chaibi, Menif, and Erasme, 2016, spectral comb is generated using two dual drive Mach Zehnder modulators. Even though two modulators are used only 42 spectral lines at maximum are generated and cost of using this method is high . Non-linearities can occur in external intensity modulators, which can cause instability in total transport system. To avoid this proper biasing of intensity, modulators can be considered (Li, Ma, Li, & Zhang, 2017). For higher data rate transmission, an Orthogonal Frequency Division Multiplexing (OFDM) will be a possible solution. This is very advantageous method where OFDM has high spectral efficiency and high data rate transmission (Djordjevic & Vasic, 2006). OFDM is chosen to be a better modulating scheme for next generation optical network. In Djordjevic and Vasic (2006), the authors used four continuous wave laser sources for WDMPON. The cost of using this method is low, because less number of laser sources are used and it is the number of users increases, more laser sources may not be a viable solution.

To overcome polarization mode dispersions and chromatic dispersions in optical fiber, OFDM is considered to be the best technique. Peak to average power ratio occur in OFDM is disadvantageous. In Shih et al., 2009, 12- tupling technique is used where only two carriers would be generated with 12 times of the distance between them. In highly nonlinear fiber, when four wave mixing is used spectral lines obtained from that technology are 103 but power variations are greater than 5 dB. When cascading FWM and self-phase modulation process in highly nonlinear fiber, then 143 spectral lines with power difference greater than 4.5 dB occurred and tone to noise ratio is considerably low (Yang, Dong, Liao, Huang, & Zhang, 2013). Frequency selective fading occurs in channel causes the design of the receiver very complex. But usage of OFDM can convert entire frequency selective fading into small flat fading channels. Flat fading can be easily overcome by the receiver. The varying intensity portions of optical signal while travelling through the fiber cause change in the refractive index of the medium. This phenomenon is called as non- linear effect of fiber (Singh & Singh, 2007). Vertical Cavity Surface Emitting Laser (VCSEL) is a semiconductor material having cavities that generates optical multicarrier. This technique uses no filters or phase shifters or external modulators but number of spectral lines generation is based on the cavities. In (Serrano, et al., 2013), using VCSEL only 32 spectral lines are occurred. Kerr non linearity effects in the fiber can also be overcome by super channels obtained from OFCG (Yang, et al., 2013). While using Dual Polarization In-phase Quadrature Modulator (DPIQM) along with frequency recirculating loop, results in optical flat comb spectrum but main disadvantage is external modulator bias controller has to be deployed in the set-up (Yang, et al., 2013). Multicarrier are also obtained using n- number of phase modulators connected in parallel and driven with separate dc and phase control at each arm (Sakamoto & Chiba, 2017). However. phase modulator generates unwanted harmonics irrespective of its driving condition. VCSEL array can also be used to replace at OLT side of passive optical network (Mun, et al., 2016), but it cannot account for large number of channels.

The main objective of proposed method is a cost effective methodology for optical multi carrier generation. The system comprises of Amplitude Modulator (AM) and a Single Drive Mach Zehnder Modulator (SDMZM), which are driven by a single RF signal generator at a frequency of 30 GHz. Here, cascading of AM and MZM provides ultra-flattened optical carriers. MZM can be used for both optical signal modulation and flat comb generation. An operating point of SDMZM has to be locked at maximum position so that it can be kept stable throughout the operation. Power ripple between carriers is less than 1 dB and spacing between each spectral tone depends on the RF frequency, used to drive the intensity modulators. Suggested system is deployed in WDM-PON at Optical Line Terminal (OLT) side. Each spectral line or tone is separated and modulated with any of the digital modulation techniques. The authors are using 4-QAM to modulate data at 10 Gbps over OFDM subcarriers. Then these subcarriers are used to modulate each spectral line. All modulated carriers are multiplexed using wavelength division multiplexer and sent over a Single Mode Fiber (SMF) over 100 Km. At receiver side demultiplexer is employed to separate each spectral line. The respective de-modulators will be used to demodulate data from the carrier signal. This technique reduces the number of laser sources on the OLT side and is very cost effective. So, the main contributions of the work are described briefly as follows:

• OFCS generation based on cascading AM and MZM.
• High efficient data transmission for WDM-PON.
• Many users can be served with higher transmission rate in next generation optical network.
• A large number of laser sources to be used on the OLT side are replaced by the OFCS generator.

A continuous wave laser source at 193.1 THz is fed to Amplitude Modulator (AM), where the line width of laser used is 10 MHz its output power is 15 dBm. The AM is driven by RF signal of about 30 GHz frequency and modulation index of AM is set to 1. The output of the amplitude modulator contains number of harmonics and those harmonics are not capable of carrying information signal. Hence the harmonics are given as input to the Mach Zehnder Modulator (MZM), which energizes the harmonics to carry large amount of data. MZM is driven by the same signal of 30 GHz of frequency. Thus, the output of MZM contains a flat comb of 51 spectral lines with least variations in its power. For generation of optical flat comb spectrum no optical filters or amplifiers are used, so this technique is considered to be cost effective technique. The block diagram of Optical Flat Comb Source (OFCS) generator is shown in Figure 1. The OFCS predominantly comprises a laser diode, amplitude modulator and single drive Mach zehnder modulator.

Figure 1. Block Diagram Representation for OFCS Generator

The continuous wave laser diode's output can be represented as,

(1)

where, E_o is the amplitude of the optical component and ω_o represents the phase of an optical component generated.

where, ω_o= 2πf_o, f_o is the frequency of laser.

The general form of output of the Amplitude modulator is given as,

(2)

where, mod (t) = 1 - m + m V_rf (t)

m is the modulation index of the Amplitude modulator and V_rf (t) is the RF driving signal and it is represented as

(3)

where, f is RF frequency. Substitute all these equations in equation (2),

(4)

By solving the above equation using binomial expressions,

(5)

(6)

Each component in equation (6) represents harmonics obtained at the output of amplitude modulator. First component represents real laser component at 193.1 THz, other components represents the respective harmonics generated from amplitude modulation with the spacing of 30 GHz.

The equation (6) can be simplified as,

(7)

The amplitude modulator is connected in cascade with the mach zehnder modulator to produce flat comb multicarrier. Each harmonic at the output of AM is enhanced to a separate carrier with the help of properly biased single drive MZM. Generally, output from SDMZM can be represented mathematically as,

(8)

The above equation describes the output of SDMZM, where V_dc is the DC bias voltage for the mach zehnder modulator and v_π is the half wave voltage, which is defined as the voltage given to MZM to provide the phase change of π radians. The product of amplitude modulator i.e., E_am(t) is fed as an input to SDMZM and modulator is driven by proper dc bias voltage and RF switching voltage. As both of these modulators are cascaded E_am(t) is multiplied with E_m(t), so the output of the total system is obtained as,

(9)

Applying Bessel functions on the above equation, E_t(t)=

(10)

Hence the above equation represents final optical flat comb spectrum, where n = 0,1,2,3,….

The equation (10) represents the spectral comb with equal power distribution over the spectral lines.

The Figure 2 represents a WDM-PON using OFCG which can entertain 51 users. Now, in this paper the authors, discuss data transmission with the help of 8 optical carriers. Each carrier can be separated with the help of 1 x 8 demultiplexer, where needed channels have to be locked in the demultiplexer at 50 GHz spacing between the channels. This 50 GHz spacing is set based on ITU standard. Each output line of demultiplexer contain single carrier. Then these spectral lines are modulated with the help of OFDM. At data modulation side, pseudo random sequence generator generates binary data, which are precoded using Quadrature Amplitude Modulator i.e., 4- QAM is used. In QAM, both amplitude and phase of the signal would be considered, where data stream is divided into two parallel sub-sequences and modulated with in-phase and quadrature phase carrier components. The outputs of QAM consist of two modulated signals and these are fed to OFDM. In OFDM higher rate data stream will be converted to larger number of lower rate data stream using serial to parallel converter. Then converted data stream would be mapped to respective symbols. Each and every symbol would be assigned to a subcarrier and the Inverse Fast Fourier Transform (IFFT) algorithm is applied at subcarriers so that time waveform of corresponding subcarriers will be obtained. Adding cyclic prefix bits would help to retrieve the data at receiver side. An in-built Digital to Analog Converter (DAC) uses different interpolation methods to convert digital stream into analog signals. Parallel to serial converter at the end will combine all parallel sub-sequences. Here OFDM is selected for several reasons such as it is resilient to interference, high spectral efficiency, immunity to selective frequency fading and simpler channel equalization. After this process, the electrical signal will be given to Dual- Drive Mach Zehnder Modulator (DD-MZM) (instead of DDMZM, SDMZM can be used) using quadrature modulator and those electrical signals modulate the input optical signal, which was selected by the 1 x 8 demultiplexer. A switching bias voltage, switching RF voltage and extinction ratio of DDMZM are set to 4 V, 4V and 30 dB respectively. Output from each of the DDMZM is multiplexed using 8 x 1 MUX and fed to an optical fiber of length 60 Km, where attenuation is adjusted to 0.2 dB/km.

Figure 2. Schematic Representation of WDM-PON using OFCG

At receiver side, a 1 x 8 D-MUX helps to separate the multiplexed channels. Then each optical channel will be detected by photo detectors. So, eight photo detectors would be used there. Photo detectors convert optical signal into corresponding electrical signal. Detected electrical signals will be processed and demodulated to obtain digital data. Firstly, an electrical amplifier boosts the electrical signal. An amplifier is used immediately after a photo detector that is because of attenuation occurred to the strength of the signal. A quadrature demodulator separates single electrical signal into in-phase and quadrature phase signals, which are given to OFDM demodulator. It does reverse operation of OFDM. Cyclic prefix is added during modulation and is removed by FFT algorithm that helps to find the original transmitted spectrum. Then, by demodulating the phase change occurred in spectrum original data word can be retrieved.

Each and every parameters of laser source and modulators are tuned in such a way that resulting spectrum would contain evenly power distributed carriers. The output of the laser shown in the Figure 3 is obtained by setting line width of the laser at 10 MHz and power at 15 dBm. RF signal generator is fixed to 30 GHz frequency and amplitude of RF signal is set to 2 a.u. The RF signal is fed to AM and MZM, where modulation index m is fixed at 1and MZM's extinction ratio is set between range of 35-40 dB and the symmetry factor at 0.955. Number of harmonics generated from MZM depend on an extinction ratio.

In Figure 3, centre carrier is obtained at 193.1 THz has 13.8 dBm power.

Figure 3. Output of Laser

Figure 4 represents the spectrum containing harmonics, in which few are capable of carrying information because the power distribution over the spectrum is uneven. To enhance these harmonics the authors send the spectrum obtained in Figure 3 to MZM as input and these are driven by the same 30 GHz RF signal.

Figure 4. Output Spectrum of AM

From Figure 5, it is noted that power excursions between each spectral tone is noted to be less than or equal to 1 dBm. Each carrier generated from this technique supports data rate for next generation network. Hence the authors have demonstrated a cost- effective technique for OFC spectrum generation, which will act as a source for WDMPON. While comparing with previous techniques (Li, et al. 2017; Sakamoto & Chiba, 2017), it is noted that our proposal gives large number of spectral tones in cascaded configuration of modulators.

Figure 5. Represents OFCS Obtained from SDMZM

The authors successfully proposed and demonstrated multiple frequency optical MMW generation and usage of those multicarrier in WDM-PON. Our proposal is the cost effective method and can be utilized in many applications such as high-speed broadband connections, 4G (especially in LTE advanced), metrology, etc. The authors consider this as very effective technique as they are not using any filters, phase shifters, attenuators and amplifiers in OFCS generation. At optical line terminal side of PON, OFCG is used and only 8 carriers are separated using demultiplexer. Using OFDM at OLT each of those carriers are modulated and transmitted via optical fiber. Each carrier is capable of carrying data at the rate of 10 Gbps and if 51 spectral tones are utilized, then rate of transmission will be 510 Gbps. Hence, the authors consider this proposal as the best and effective way of approach for reliable communication.